18,223 research outputs found
Soft X-ray emission in kink-unstable coronal loops
Solar flares are associated with intense soft X-ray emission generated by the
hot flaring plasma. Kink unstable twisted flux-ropes provide a source of
magnetic energy which can be released impulsively and account for the flare
plasma heating. We compute the temporal evolution of the thermal X-ray emission
in kink-unstable coronal loops using MHD simulations and discuss the results of
with respect to solar flare observations. The model consists of a highly
twisted loop embedded in a region of uniform and untwisted coronal magnetic
field. We let the kink instability develop, compute the evolution of the plasma
properties in the loop (density, temperature) without accounting for mass
exchange with the chromosphere. We then deduce the X-ray emission properties of
the plasma during the whole flaring episode. During the initial phase of the
instability plasma heating is mostly adiabatic. Ohmic diffusion takes over as
the instability saturates, leading to strong and impulsive heating (> 20 MK),
to a quick enhancement of X-ray emission and to the hardening of the thermal
X-ray spectrum. The temperature distribution of the plasma becomes broad, with
the emission measure depending strongly on temperature. Significant emission
measures arise for plasma at temperatures T > 9 MK. The magnetic flux-rope then
relaxes progressively towards a lower energy state as it reconnects with the
background flux. The loop plasma suffers smaller sporadic heating events but
cools down conductively. The total thermal X-ray emission slowly fades away
during this phase, and the high temperature component of emission measure
distribution converges to the power-law distribution . The
amount of twist deduced directly from the X-ray emission patterns is
considerably lower than the maximum magnetic twist in the simulated flux-ropes.Comment: submitted to A&
Measuring Baryon Acoustic Oscillations with Millions of Supernovae
Since type Ia Supernovae (SNe) explode in galaxies, they can, in principle,
be used as the same tracer of the large-scale structure as their hosts to
measure baryon acoustic oscillations (BAOs). To realize this, one must obtain a
dense integrated sampling of SNe over a large fraction of the sky, which may
only be achievable photometrically with future projects such as the Large
Synoptic Survey Telescope. The advantage of SN BAOs is that SNe have more
uniform luminosities and more accurate photometric redshifts than galaxies, but
the disadvantage is that they are transitory and hard to obtain in large number
at high redshift. We find that a half-sky photometric SN survey to redshift z =
0.8 is able to measure the baryon signature in the SN spatial power spectrum.
Although dark energy constraints from SN BAOs are weak, they can significantly
improve the results from SN luminosity distances of the same data, and the
combination of the two is no longer sensitive to cosmic microwave background
priors.Comment: 4 pages, 3 figures, ApJL accepte
Comments on the Quantum Potential Approach to a Class of Quantum Cosmological Models
In this comment we bring attention to the fact that when we apply the
ontological interpretation of quantum mechanics, we must be sure to use it in
the coordinate representation. This is particularly important when canonical
tranformations that mix momenta and coordinates are present. This implies that
some of the results obtained by A. B\l aut and J. Kowalski-Glikman are
incorrect.Comment: 7 pages, LaTe
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